1. Field of the Invention
The present invention pertains to a DC-DC converter.
2. History of the Prior Art
A typical prior art DC-DC converter has been proposed as disclosed in U.S. Pat. No. 3,818,484 issued June 18, 1974 to T. Nakamura et al., and assigned to the assignee of this application. As shown in FIG. 1, the DC-DC converter is of the Schenkel type wherein enable signals S of rectangular waveform are entered into a two-phase CMOS driver DR supplied with a DC power source voltage -V.sub.ss so that two-phase enable signals Q and Q are generated from the driver DR. By switching of diodes D.sub.1, D.sub.2, . . . D.sub.n, capacitors C.sub.1, C.sub.2, . . . C.sub.n are charged with sequentially multiplied voltages. As a consequence, a DC voltage of -nV.sub.ss is developed at a load resistor R.sub.L.
Nevertheless, a problem is experienced in the voltage multiplier employing the switching function of these diodes D.sub.1, D.sub.2, . . . D.sub.n, in that the conversion efficiency will decline markedly when the DC source voltage -V.sub.ss falls to an appreciable extent with reference to the forward voltage of the diodes D.sub.1, D.sub.2, . . . D.sub.n. For example, solid state wristwatches are powered with a relatively low voltage of about 175 V. In the case where the above shown circuit is utilized as a DC-DC converter for solid state watches, employment of conventional P-N junction type diodes is not accepted for practical use because their forward voltage can not be ignored as compared with the power source voltage with the result of remarkably reduced efficiency. However, employment of the Schottkey barrier type diodes is acceptable because of forward lower voltages than that of the conventional P-N junction type diodes. In this instance, high efficiency of about 80% is ensured when using the converter in solid state wristwatches.
Meanwhile, difficulties will be encountered for the Schottkey barrier type diodes (but not for the conventional P-N junction type) in incorporating the diodes D.sub.1, D.sub.2, . . . D.sub.n into an IC or LSI semiconductor chip containing a timekeeping circuit. It is a requirement for solid state wristwatches to have low cost and a small amount of circuit components. However, Schottkey barrier type diodes have to be discrete components, separate from the IC and LSI semiconductor chip of the watch, which adds to the size and cost of the watch.